Perpendicular magnetic recording (PMR) has become the mainstream technology for disk drive applications beyond 200 Gbit/in2, replacing longitudinal magnetic recording (LMR) devices. Due to the continuing reduction of transducer size, high moment soft magnetic thin films with a Bs above 22 kG are required for write head applications. A PMR head which combines the features of a single pole writer and a double layered media has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density. In particular, a shielded pole head can provide a large head field gradient at the trailing side due to the presence of a trailing shield and substantially improve the write performance.
Perpendicular recording demands increasingly smaller pole tip size in the magnetic recording heads to achieve higher data density on the magnetic medium. However, prior studies have shown that the perpendicular direction shape anisotropy in the smaller writer pole tips may lead to serious pole erasure (PE). In other words, a remanent magnetic field can exist in the write pole tip after the write current is turned off thereby leading to data erasure in the magnetic medium during non-write operations. References related to this subject are (1) K. Hirata et al., “A study of pole material properties for pole erasure suppression in perpendicular recording heads”, J. Magn. Magn. Mat., Vol. 287, pp. 352-356, November 2004; (2) Y. Zhou et al., “Perpendicular write head remanence characterization using a contact scanning recording tested”, J. Appl. Phys., Vol. 97, 10N903 (2005); and (3) Y. Zhou and J.-G. Zhu, “Dependence of the pole tip remanence on the medium magnetization state underneath the trailing shield of a perpendicular write head”, J. App. Phys. Vol. 97, 10N518 (2005).
To alleviate the PE problem, two approaches are described by D. Bai et al. in “Writer pole tip remanence in perpendicular recording”, IEEE Trans. Magn., Vol. 42, p. 473 (2006) and typically involve making the writer head into a laminated multi-layered structure through dry film deposition methods. According to a first method, making the write head into a laminated multi-layer structure where thicker magnetic layers are separated by thinner non-magnetic layers, the magneto-static coupling field between the adjacent layers through the edge charges will help the write pole maintain a near zero net magnetic moment when the write current is turned off. The inter-layer magneto-static coupling prefers an anti-parallel magnetization orientation of adjacent magnetic layers which leads to charge cancellation between the adjacent magnetic layers and to a net zero magnetic charge from the write pole tip as a whole. The second approach is similar to the first. However, the thin spacer layer between neighboring magnetic layers is chosen from a specific metal such as Ru or Cr and has a specific thickness. Thus, due to the Ruderman-Kittel-Kasuya-Yoshida (RKKY) interaction between localized moments mediated by the conduction electrons of the spacer metal, an AFM exchange coupling between the adjacent magnetic layers can be established. This AFM coupling field is usually much higher than the magneto-static coupling field involved in the first approach and therefore is more effective in reducing the PE field in perpendicular write heads.
Unfortunately, reducing the PE field with either magneto-static coupling or AFM coupling results in a loss in writability (write field) and particularly near the trailing edge of the write pole tip. The coupling field that minimizes PE also reduces the write field through the same interactions. In addition, a non-magnetic spacer layer causes a further decrease in write field because of a reduced volume of magnetic material in the write pole. Thus, a trade off exists between reducing the PE field during non-write operations and maintaining a strong write field during the writing process in state of the art PMR technology.
It is known in the prior art that FeRh has an abrupt first order transition from an AFM phase to a FM phase without structural change at a rather low temperature of 330° K to 350° K. This abrupt phase change is thought to be associated with a CsCl type bodied centered cubic (bcc) Fe50Rh50 alloy. References include the following: J. Lommel and J. Kouvel, “Effects of mechanical and thermal treatment on the structure and magnetic transitions of FeRh”, J. Appl. Phys., Vol. 38, pp. 1263-1264, March 1967; Y. Ohtani and I. Hatakeyama, “Antiferro-ferromagnetic transition and microstructural properties in a sputter deposited FeRh thin film system”, J. Appl. Phys., Vol. 74, p. 3328, September 1993; C. Paduani, “Magnetic properties of Fe—Rh alloys”, J. Appl. Phys., Vol. 90, p. 6251, December 2001; and J. Thiele et al., “Magnetic and Structural Properties of FePt—FeRh Exchange Spring films for Thermally Assisted Magnetic Recording Media”, IEEE Trans. Magn., Vol. 40, p. 2537 (2004).
In U.S. Patent Application Publication No. 2003/0108721 and in a publication by S. Koyama et al., “Reduction of coercivity in FePt—FeRh bilayer films by heating”, IEEE Trans. Magn., Vol. 41, p. 2854 (2005), the AFM-FM phase transition is shown to increase or decrease by controlling the seed layer and employing a FeRhX alloy where X can be Pd, Pt, Ir, etc. Compositional control of the FeRhX material can also produce as narrow as a 10° K to ˜20° K difference in AFM=>FM transition temperature during heating and FM=>AFM transition temperature during cooling. In related U.S. Pat. No. 6,834,026, a TAMR disk is described that is comprised of a bilayer of an FM layer and a layer that switches between FM and AFM states by a temperature change.
Referring to FIG. 1, an illustration of AFM=>FM transition curves taken from U.S. Patent Application Publication No. 2005/0281081 is shown for different FeRh alloys with various compositions. This composition modifiable switching temperature and narrow transition window theoretically make the AFM/FM phase control of FeRh alloys easy to realize during actual write head operations.
The prior art references mentioned above that involve FeRh or FeRhX alloys are mainly focused on magnetic recording medium applications where an FeRh alloy layer is part of the magnetic recording layer that also contains another hard magnetic material which has a very high anisotropy. A high anisotropy in the hard magnetic material is needed to enhance the recorded bit's thermal stability when the bit physical size is in the sub-micron range. However, the high anisotropy also makes the magnetization in the recording layer not easy to reverse during a write process. Thermal heating of the FeRh induces an AFM to FM transition and the resulting FM state of the FeRh is a soft material with very low anisotropy. With the FM exchange between the hard magnetic layer and the FeRh layer, the FM phase of FeRh enables an easier reversal of magnetization in the recording layer during a write process. Heat assisted magnetic recording, also known as HAMR, has been a topic of major interest in the prior art. A similar idea is utilized in a MRAM application in U.S. Patent Application Publication No. 2005/0281081 where a data storage layer is also a hard material abutted with a FeRh layer. The switching current field is easier to reverse by means of a thermally induced FM phase in the FeRh layer.
Formation of a FeRh layer in the prior art usually involves relatively high temperature (>400° C.) during deposition as described in the aforementioned Thiele reference, and typically includes an after-deposition thermal treatment. This process which generally results in a bcc CsCl type FeRh crystalline structure is considered too harsh for commercial recording head fabrication. More recent methods are shown to form a bcc FeRh structure with lower substrate temperature and shorter annealing time and involve proper selection of seed layer and substrate as described by S. Maat et al. in “Temperature and field hysteresis of the antiferromagnetic-to-ferromagnetic phase transition in epitaxial FeRh films”, Phys. Rev. B, Vol. 72, p. 214432-1 (2005), or employ an equi-atomic composition of Fe and Rh as described by S. Hashi et al., “A large thermal elasticity of the ordered FeRh alloy film with sharp magnetic transition”, IEEE Trans. Magn., Vol. 40, p. 2784 (2004).
Replacing a small percentage of the Rh atoms in the FeRh lattice with an equal amount of Pd, Pt, or Ir atoms could help to reduce the annealing temperature and annealing time during a FeRhX alloy deposition without a significant reduction in magnetic moment. With Pd, Pt, or Ir addition, the resulting FeRhPd, FeRhPt, or FeRhIr alloys can also reduce the FM=>AFM transition temperature down to about 100° C., making it more practical for device applications. Besides conventional deposition and after-deposition annealing, a multi-layer FeRh structure with mono-layer level Fe/Rh thickness and a bcc-like structure has been formed as shown by M. Tomaz et al., “Fe/Rh (100) multilayer magnetism probed by x-ray magnetic circular dichroism”, Phys. Rev. B, Vol. 57, p. 5474, September 1997. A FeRh structure with an intrinsic AFM state is described by D. Spisak and J. Hafner in “Structural, magnetic, and chemical properties of thin Fe films grown on Rh (100) surfaces investigated with density functional theory”, Phys. Rev. B, Vol. 73, p. 155428 (2006). These results may provide an alternative way to synthesize a bcc FeRh thin film layer for AFM/FM transition.
In U.S. Pat. No. 6,410,170, a FeRh layer is used in a pole or shield structure of a write head.
A conventional PMR write head as depicted in FIG. 2 typically has a main pole layer 10 with a pole tip 10t at an air bearing surface (ABS) 5 and a flux return pole (opposing pole) 8 which is magnetically coupled to the write pole through a trailing shield 7. Magnetic flux in the main pole layer 10 is generated by coils 6 and passes through the pole tip into a magnetic recording media 4 and then back to the write head by entering the flux return pole 8. The write pole concentrates magnetic flux so that the magnetic field at the write pole tip 10t at the ABS is high enough to switch magnetizations in the recording media 4. A trailing shield 7 is added to improve the field gradient in the down-track direction.
Referring to FIG. 3, a top view is shown of a typical main pole layer 10 that has a large, wide portion called a yoke 10y, a narrow rectangular portion 10p called a write pole that extends a neck height (NH) distance y from the ABS plane 5-5 to a plane 3-3 parallel to the ABS where the pole intersects a flare portion 10f at the neck 12. The flare portion 10f adjoins the yoke along a plane 4-4 and flares outward from the plane 3-3 at an angle θ from a dashed line 11 that is an extension of one of the long rectangular sides of the write pole 10p. PMR technologies require the pole 10p at the ABS to have a beveled shape (as viewed from the ABS) so that the skew related writing errors can be suppressed. A write head may also have a stitched pole configuration as described in U.S. Pat. No. 6,826,015.
One disadvantage of prior art AFC lamination schemes is that the coupling strength of a FeCo/Ru/FeCo configuration or the like is typically large and this type of AFC lamination will inevitably cause a large anisotropy field and low magnetic moment under a low field. Although the coupling strength can be lowered by using a thicker non-magnetic layer (increasing Ru thickness from 7.5 to about 18 Angstroms, for example), the magnetic moment will be diluted as the non-magnetic content in the FeCo/Ru/FeCo stack is increased. Therefore, an improved lamination scheme for a write pole is needed that enables a high magnetic moment while simultaneously providing a mechanism to reduce remanence.